“Nicea put the finishing touches, sealed the deal. There were still debates going on before that.”

There were debates still going on after that too. Constantine and Nicean council were not more significant than other councils and Hus, Luther, or Calvin. If there was something unique about Nicea, it was the fact that from now on bishops could use Roman army to fight whom they deemed heretics and pagans.

“Mithraism may have the closest resemblances”

I would not say so. There is almost nothing in original Mithraic cult that would correlate with Jesus. Mithras was born from a rock (no virgin mother and no nativity story) and there is no clear indication of his death let alone resurrection. He was not called “the good shepherd” – he was a mediator between good and evil gods (not between man and God). He was also originally not the same as Sol Invictus. And so on and so forth. All in all, and if anything, Christianity influenced Mithraism not the other way round (which may also explain why Constantine had no problems with making Christianity official as Mithraism was popular among his soldiers).

With that being said, I would personally go with Tammuz aka Dumuzid (god of milk and shepherds, his plant was date palm, regularly resurrects – spends half of the year in Heaven and another half in Underworld, etc.)

“I also am reluctant to dismiss Paul so readily. I think he needs exploring in greater depth.”

Oh, I guess I didn't make myself clear – I would dismiss Saul completely. Even to the point of filtering out his influence from Gospels.